It can make a big difference for wattage depending if you are using a valve amp or solid state. You will need a higher wattage solid state amp to reach the same volume levels of a lower wattage valve amp.

The general rule i go by is for live performances with a band you would need at least a 50W valve amp or 100W solid state amp if you didn't plan on having it mic'd up.

If you are planning on having the amp mic'd up then you can get away with using lower wattage amps.